1. Field of the Invention
This invention relates to an optical system for recording and/or reproducing an optical recording medium and, more particularly, to an optical pickup and to a recording and/or reproducing apparatus for an optical recording medium capable of writing and reading out data for plural recording mediums using plural recording and/or reproducing wavelengths.
2. Description of Related Art
Recently, recording mediums, such as CD (Compact Disc), MD (Mini-Disc) and DVD (Digital Versatile Disc), are required to be further increased in capacity. In order to meet this demand, a variety of techniques have so far been proposed. On the other hand, a sole medium is required to be able to freely record and/or reproduce variegated data, such as, for example, music contents data, image contents data or data for computers. Above all, a novel disc format employing the laser of a wavelength band of 405 nm, referred to below as a Blue-Ray Disc (BD), is stirring up notice as representing the next-generation recording technique.
In developing universally employed mediums, compatibility and matching performance between a recording and/or reproducing apparatus for a new medium and one for an old medium, are crucial, such that newly developed recording and/or reproducing apparatus are desirably capable of exploiting conventional resources, such as DVD or CD. However, it is not that easy to implement an apparatus which will provide for compatibility between mediums having disc structures and concomitant laser specifications different from each other.
The simplest method is to provide different optical systems and to switch between dedicated objective lenses from one wavelength to another. However, a switching mechanism for switching between plural sorts of the objective lenses is needed in such case, thus leading to increased cost. An actuator becoming bulky is deterrent to size reduction of the apparatus. For this reason, a multiple wavelength interchange optical system, in which part of the optical system, such as an objective lens, is co-owned, has been in use. However, in such case, spherical aberration generated increases in proportion to the thickness of a protective substrate designed to protect the recording surface, with the result that, with the conventional single lens, it is difficult to condense light beams of different wavelengths on the recording surfaces through protective substrates of different thicknesses in a substantially aberration-free fashion.
Thus, as a system for correcting the spherical aberration, in a conventional dual wavelength inter-changing system for coping with recording and/or reproduction of two recording mediums, having different protective substrate thicknesses, a diffraction system for allowing a specified wavelength to be incident with diffraction, by employing an optical element, such as a diffraction element, or without diffraction, on an objective lens, or a multiplication factor converting system in which the optical path length is rendered finite to change the multiplication factor, has so far been used.
With the dual wavelength inter-changing system, the diffractive element may be designed, by combining a certain curved lens surface with the diffractive element (two degrees of freedom) such as to satisfy the optimum value of the combination of the two different protective substrate thicknesses and the recording and/or reproducing wavelengths (two degrees of freedom), thereby solving the above problem.
However, if it is tried to implement three-wavelength compatibility among DVD, CD and e.g. the aforementioned BD of the new format, there are three combinations each of the protective substrate thicknesses and wavelengths that need to be optimized, with the result that, with the technology employing a sole diffractive element, the degrees of freedom fall short and hence the transmission diffraction efficiency and the angle of diffraction cannot be optimized simultaneously with ease. In the former system by diffraction, the spherical aberration among the three wavelengths can be corrected by employing plural angles of diffraction. However, in this case, the transmission diffraction efficiency is lowered significantly depending on the shape of the diffraction grating and grating characteristics. The latter system by multiplication factor conversion, laser light beams of different wavelengths need to be arranged at different multiplication factor positions, so that the lasers cannot be placed with ease in one and the same casing, in a manner unmeritorious to reducing the apparatus size. In addition, the performance of the objective lens in movement along the tracking direction is lowered significantly.
In case of the BD, it is relatively easy to design an objective lens, achieving the light utilization efficiency not lower than 95%, in case a dedicated objective lens, intended to be used for the BD by itself, is used. However, if it is tried to provide for compatibility with the DVD (Digital Versatile Disc) and CD (Compact Disc), as conventional resources, many problems arise. Here, a case of extending the dual wavelength compatible optical system for the BD and the DVD, for which compatibility has already been achieved to a certain extent, for use with a light beam for a CD, is explained, as an example.
As the dual wavelength compatible optical system for the BD and the DVD, there is, for example, such a system which is based on a light beam for recording and/or reproducing a BD and in which a light beam for BD incident on an objective lens is of an infinite system, while a light beam for CD and DVD being of a finite system. The infinite system means such a system in which a light beam falls on an objective lens from an infinitely remote distance, that is, as a collimated light beam on the objective lens, whilst the finite system means such a system in which a light beam falls as divergent light on the objective lens. The compatibility system by the finite and the infinite systems is meritorious in that the action of diffraction is not used so that light utilization efficiency is satisfactory, and also in that the focal length can be varied depending on the degree of divergence of the light beam incident on the objective lens and hence an optimum working distance can be achieved. However, since the light beam falls on the objective lens as divergent light (light of the finite system), there is not sufficient margin for transverse deflection of the objective lens with respect to the optical axis, with the result that the tracking margin tends to be affected significantly.
If, in a BD-DVD dual wavelength compatible optical system, a light beam for recording and/or reproducing a BD is used as a base beam, and an optical component, optimized to generate order-one light beams for both the BD and the DVD, is used, approximately 90% order-one diffracted light is generated for a light beam for BD (405 nm), however, only approximately 70% order-one diffracted light may be generated for a light beam for DVD (655 nm). Even though the system is satisfactory from the perspective of aberration characteristics, the transmission diffraction efficiency for the light beam for DVD is that low to degrade the light utilization efficiency. Moreover, it is difficult with this system to provide for compatibility with the light beam for CD (785 nm).
If, in a BD-DVD dual wavelength compatible optical system, a light beam for recording and/or reproducing a BD is used as a base beam, and an optical component, optimized to generate order-two diffracted light for BD and order-one diffracted light for DVD, is used, the transmission diffraction efficiency of approximately 90% may be achieved for order-two diffracted light for BD and order-one diffracted light for DVD, thus achieving satisfactory light utilization efficiency. However, this system suffers from marked wavelength dependency of the spherical aberration and difficulty in achieving compatibility with the light beam for CD (785 nm).
FIG. 1 schematically shows characteristics in case of employing the aforementioned BD-DVD dual wavelength compatible optical system. In FIG. 1, the ordinate and the abscissa denote thicknesses of the protective substrates protecting the recording surfaces of the optical discs and wavelength bands of light beams used, respectively. Since the amounts of the spherical aberration generated are proportional to the thicknesses of the protective substrates and the angles of diffraction are proportionate to the wavelengths, FIG. 1 also shows the relationship between the amounts of the spherical aberration and the angles of diffraction for respective wavelengths.
As may be seen from FIG. 1, if the order-one diffracted light for CD is incident on an optical system employing a light beam for recording and/or reproducing the BD as a base beam and an optical component optimized to generate order-one diffracted light beams for both BD and DVD (BD/DVD/CD=order one/order one/order one), there is generated, in the BD-DVD compatible optical system, there is produced spherical aberration SA111 that cannot be compensated for the light beam for CD, referred to herein as residual spherical aberration.